Actually either would work, and pea gravel is much easier to shovel, if you get a truck load of it, it shovels right into a wheel barrel...road base is 3 or 5 sided (so it nests & interlocks and provides a stable road base.) and road base is heavy !
Mounding the rock is possible with road base, no so with pea gravel, which will level out after a while.
If the rock is mounded, rain would run off, but you still must wash it off after a while it is full of poo.....I use a strong wash of water from the hose...an odd hobby of mine : washing poo...
If the poo builds up, after a while it can become so acidic that it will cause burns to the bird's feet...we see foot rot like this with pigs, also...they were not meant to constantly be standing in their own feces.
The neighbors here have 4 large hogs in a very small pen, in MUCK up to their bellies...contantly.
It is sad, and not a good idea for humans to eat these poor critters either.
They probably have quite a bad staph infection(s) going on in their feet.................
Pigs hate their own poo.....and do not care for mud either, unless it is hot weather, and CLEAN poo-free mud.
Thanks Chickielady, do appreciate all the info. None of us want wet, mucky feet/bodies. Land slopes away from where I'm standing in photo, so going to suggest they take advantage of that too.